The present invention relates to a color image forming apparatus and method for forming a color image by repeating a process for a plurality of color inks. In particular, the present invention relates to a color image forming apparatus and method comprising the steps of forming a latent image on an image bearing member, then developing the latent image with a corresponding color ink to form a color image, and finally transferring the color image onto a transfer member such as recording medium or intermediate ink receptive blanket.
Recently, environmentally friendly water-based ink has received great attention as a coloring material used for an image forming device such as copy machine or printer.
Conventionally, various image forming methods using the water-based ink have been proposed. For example, EP 0 697 290 discloses a method in which aqueous printing ink is applied to a waterless printing plate having a hydrophobic non-image surface area and a hydrophilic image area. The image surface area is wetted by the ink while the ink is repelled from the non-image surface area. JP 09-131914 A discloses a method in which an ink image carrier body having a hydrophilic layer on which a water repellent liquid is formed. The water repellent liquid is then selectively resolved to form a latent image. U.S. Pat. No. 6,027,852 discloses a method in which a printing plate having a surface formed with a material such as photochromic compound. The contact angle of the surface with water is selectively changed by photo-irradiation to form a latent image.
However, in those image forming methods, since ink surface tension influences greatly the wetting of the ink onto an image bearing member such as printing plate, it is necessary to limit the surface tension value of the ink to be used. In addition, with a color image forming device, in which second ink is superimposed on first ink to form an image, the difference in the surface tension between the inks may create problems such as bleeding, trapping failure, or backtrapping. The xe2x80x9cfirst inkxe2x80x9d refers to the ink that is transferred onto a transfer member (such as recording medium or intermediate ink receptive blanket) firstly. The xe2x80x9csecond inkxe2x80x9d refers to the ink that is transferred onto the transfer member onto which the first ink has been transferred.
Therefore, the object of the present invention is to provide a color image forming apparatus and method, allowing lower bleeding property, higher trapping property, and lower backtrapping property.
Herein, the term xe2x80x9cbleeding propertyxe2x80x9d is used to designate the degree to which the first ink and the second ink diffuse or mix into each other. The term xe2x80x9ctrapping propertyxe2x80x9d is used to designate the degree to which the second ink is transferred onto the first ink. The term xe2x80x9cbacktrapping propertyxe2x80x9d is used to designate the degree to which the first ink already on a transfer member is retransferred or offset onto an image bearing member bearing the second ink.
To achieve the above object, an image forming apparatus according to the present invention comprises an image bearing member; a latent image forming device for forming first and second latent images on the image bearing member; a developing device for developing the first latent image with first ink to form a first ink image, and developing the second latent image with second ink to form a second ink image; and a transferring device which transfers the first ink image onto a transfer member and then the second ink image onto said transfer member. The surface tension of the first ink is set higher than that of the second ink.
In another aspect of the image forming apparatus according to the present invention, the image bearing member includes a ferroelectric layer for forming a latent image therein.
In another aspect of the image forming apparatus according to the present invention, the first and second inks are water-based inks.
In another aspect of the image forming apparatus according to the present invention, the surface tension difference between the first and second inks is equal to or less than 15 dyne/cm.
An image forming method according to the present invention comprises the steps of forming first and second latent images; developing the first latent image with first ink to form a first ink image, and developing the second latent image with second ink to form a second ink image; and transferring the first ink image onto a transfer member and then the second ink image onto the transfer member. The surface tension of the first ink is set higher than that of the second ink.
In another aspect of the image forming method according to the present invention, the first and second inks are water-based inks.
In another aspect of the image forming method according to the present invention, the surface tension difference between the first and second inks is equal to or less than 15 dyne/cm.
Another image forming apparatus comprises first and second image forming units. The first image forming unit includes a first image bearing member; a first latent image forming device for forming a latent image on the first image bearing member; a first developing device for developing the latent image formed on the first image bearing member with first ink to form a first ink image; and a first transferring device for transferring the first ink image onto a transfer member. The second image forming unit includes a second image bearing member; a second latent image forming device for forming a latent image on the second image bearing member; a second developing device for developing the latent image formed on the second image bearing member with second ink to form a second ink image; and a second transferring device for transferring the second ink image onto the transfer member onto which the first ink image has been transferred. The surface tension of the first ink is set higher than that of the second ink.
In another aspect of the image forming apparatus according to the present invention, each of the first and second image bearing members includes a ferroelectric layer for forming a latent image therein.
In another aspect of the image forming apparatus according to the present invention, the first and second inks are water-based inks.
In another aspect of the image forming apparatus according to the present invention, the surface tension difference between the first and second inks is set equal to or less than 15 dyne/cm.